1. Field of the Invention
The present invention relates to image processing, and, in particular, to computer-implemented processes, apparatuses, and computer programs for converting the color format of image signals.
2. Description of the Related Art
Different color formats are used to represent images in computer-based image processing systems. Standard computer monitors use a red-green-blue (RGB) color format for displaying image and graphics signals. For example, in RGB24 format, each image pixel is represented by three 8-bit component values representing the colors red, green, and blue, respectively. The RGB24 format Supports 2.sup.24 different colors.
Many computer operating systems, such as the Microsoft.RTM. Windows.TM. operating system, provide a palette of colors that are made available to applications running under those operating systems. The palette may be defined as a color lookup table (CLUT) which provides a mapping from a CLUT index to a particular RGB24 color. For example, 8-bit CLUT8 indices may be used to define and access up to 256 different colors in a lookup table. The particular colors in the CLUT may be defined by the operating system, an application running under the operating system, or both, such that the operating system may define and reserve some of the colors in the palette and allow applications to define the rest.
Another set of color formats used in image processing is based on the 3-component YUV color system, in which Y represents the luminance component and U and V represent chrominance components. The YUV color system is used extensively in image compression applications, because it has been found to provide greater degrees of compression than, for example, the RGB color system. One such color format is the YUV9 (or YUV4:1:1) format. In this color format, each (4.times.4) block of image pixels is represented by a (4.times.4) block of 8-bit Y components, a single 8-bit U component, and a single 8-bit V component. As a result, each (4.times.4) pixel block is represented by (16.times.8+8+8) or 144 bits, for an of 9 bits per pixel. Thus, the name YUV9. Like the RGB24 format, the YUV9 format provides 224 different colors.
When decompressing YUV9 images for display on an RGB24 monitor, the decoded signals are preferably converted from YUV9 format to a CLUT format. The CLUT signals may then be transmitted to the graphics adapter card which uses the CLUT to convert the signals from CLUT8 format to RGB24 format for display on the monitor. What is needed is an efficient method for converting image signals from YUV9 format to CLUT format.
It is accordingly an object of this invention to provide an efficient method for converting image signals from YUV9 format to CLUT format. 10 Further objects and advantages of this invention will become apparent from the detailed description of a preferred embodiment which follows.